Subject: [mg61278] Re: How to reverse sign on the y's in a list of (x, y)'s?

From: "Bill White" <minutiae at gmail.com>

Date: Fri, 14 Oct 2005 05:54:39 -0400 (EDT)

References: <dikt77$4tm$1@smc.vnet.net>

Sender: owner-wri-mathgroup at wolfram.com

Here's a simple-minded approach (to help me learn about lists :-).
First, a simple list:
list = {{a, 1}, {b, 2}, {c, 3}}
Note that we can grab the elements in question with this:
list[[All, 2]]
We can make them negative by Map'ing Minus over them:
Minus /@ list[[All, 2]]
And the Help Browser says, "You can always reset one or more pieces of
a list by doing an assignment like m[[ \[Ellipsis] ]] = value."
So we should be able to change the sign of the 2nd elements with this:
list[[All, 2]] = Minus /@ list[[All, 2]]
Indeed, here is the new list:
In[18]:= list
Out[18]= {{a,-1},{b,-2},{c,-3}}
--------------------------------------
Summary:
In[19]:= list={{a,1},{b,2},{c,3}}
Out[19]= {{a,1},{b,2},{c,3}}
In[20]:= list[[All,2]]=Minus/@list[[All,2]]
Out[20]= {-1,-2,-3}
In[21]:= list
Out[21]= {{a,-1},{b,-2},{c,-3}}
Cheers